Extreme ultraviolet soft x-ray projection lithographic method and mask devices

ABSTRACT

The present invention relates to reflective masks and their use for reflecting extreme ultraviolet soft x-ray photons to enable the use of extreme ultraviolet soft x-ray radiation projection lithographic methods and systems for producing integrated circuits and forming patterns with extremely small feature dimensions. The projection lithographic method includes providing an illumination sub-system for producing and directing an extreme ultraviolet soft x-ray radiation λ from an extreme ultraviolet soft x-ray source; providing a mask sub-system illuminated by the extreme ultraviolet soft x-ray radiation λ produced by the illumination sub-system and providing the mask sub-system includes providing a patterned reflective mask for forming a projected mask pattern when illuminated by radiation λ. Providing the patterned reflective mask includes providing a Ti doped high purity SiO 2  glass wafer with a patterned absorbing overlay overlaying the reflective multilayer coated Ti doped high purity SiO 2  glass defect free wafer surface that has an Ra roughness≦0.15 nm. The method includes providing a projection sub-system and a print media subject wafer which has a radiation sensitive wafer surface wherein the projection sub-system projects the projected mask pattern from the patterned reflective mask onto the radiation sensitive wafer surface.

This application is a Continuation of application Ser. No. 09/615,621,filed Jul. 13, 2000, now U.S. Pat. No. 6,465,272.

BACKGROUND OF THE INVENTION

This application claims the benefit of U.S. Provisional Application,Ser. No. 60/145,057, filed Jul. 22, 1999 entitled Extreme UltravioletSoft X-Ray Projection Lithographic Method and Mask Devices, by Claude L.Davis, Robert Sabia and Harrie J. Stevens.

U.S. Provisional Application, Ser. No. 60/149,840, filed Aug. 19, 1999entitled Extreme Ultraviolet Soft X-Ray Projection Lithographic Methodand Mask Devices, by Claude L. Davis, Kenneth E. Hrdina, Robert Sabiaand Harrie J. Stevens.

FIELD OF THE INVENTION

The present invention relates generally to projection lithographicmethods and systems for producing integrated circuits and formingpatterns with extremely small feature dimensions. The present inventionrelates particularly to extreme ultraviolet soft x-ray based lithographyand reflective masks which reflect extreme ultraviolet soft x-rayradiation and form pattern images that are utilized to form circuitpatterns. The present invention relates to reflective masks and theiruse for reflecting extreme ultraviolet soft x-ray photons to enable theuse of extreme ultraviolet soft x-ray radiation for lithography thatsurpasses current optical lithography circuit feature dimensionperformance.

TECHNICAL BACKGROUND

The use of extreme ultraviolet soft x-ray radiation provides benefits interms of achieving smaller feature dimensions but due to the nature ofthe radiation, it presents difficulties in terms of manipulating anddirecting such wavelengths of radiation and has delayed the commercialmanufacturing use of such radiation. Current optical lithography systemsused in the manufacturing of integrated circuits have progressed towardsshorter optical wavelengths of light, such as from 248 nm to 193 nm to157 nm, but the commercial use and adoption of extreme ultraviolet softx-rays has been hindered. Part of this slow progression to very shortwavelengths of radiation such as in the 15 nm range, has been due to thelack of economically manufacturable reflective mask wafers that canwithstand the exposure to such radiation while maintaining a stable andhigh quality circuit pattern image. For the benefits of extremeultraviolet soft x-rays to be utilized in the manufacturing ofintegrated circuits, there is a need for a stable glass wafer thatallows for direct deposition of reflective coatings to the surface ofthe glass wafer.

As noted from U.S. Pat. No. 5,698,113, current extreme ultraviolet softx-ray lithographic systems are extremely expensive. U.S. Pat. No.5,698,113 tries to address such high costs by trying to recover thesurfaces of multilayer coated substrates by etching the multilayerreflective coatings from substrate surfaces of fused silica and ZERODURtype aluminosilicate glass-ceramics, even though such etching degradesthe substrate surface.

The present invention provides for an economically manufactured maskwafer that is stable, ready for direct coating and receptive toreceiving multilayer reflective coatings and provides an improvedextreme ultraviolet soft x-ray based projection lithographymethod/system. The present invention economically provides for improvedmask wafer performance and stability without the need to recycle themask wafer surface which has been shown to reduce performance and thereflectivity of the mask. The present invention provides a stable highperformance reflective mask with the reflective multilayer coatingdirectly deposited on the finished glass surface, and avoids costly andcumbersome manufacturing steps and intermediate layers between the glasssubstrate surface and the reflective multilayer coating.

SUMMARY OF THE INVENTION

One aspect of the present invention is a projection lithographicmethod/system for producing integrated circuits with printed featuredimensions less than 100 nm that includes providing an illuminationsub-system for producing and directing an extreme ultraviolet soft x-rayradiation λ from an extreme ultraviolet soft x-ray source. The methodfurther includes providing a mask sub-system illuminated by the extremeultraviolet soft x-ray radiation λ produced by the illuminationsub-system and providing the mask sub-system includes providing apatterned reflective mask for forming a projected mask pattern whenilluminated by radiation λ. Providing the patterned reflective maskincludes providing a Ti doped high purity SiO₂ glass wafer with apatterned absorbing overlay overlaying the reflective multilayer coatedTi doped high purity SiO₂ glass defect free wafer surface that has an Raroughness≦0.15 nm. The method further includes providing a projectionsub-system and an integrated circuit wafer which has a radiationsensitive wafer surface wherein the projection sub-system projects theprojected mask pattern from the patterned reflective mask onto theradiation sensitive wafer surface.

In another aspect, the present invention includes a method of making aprojection lithographic system and a method of projection lithographythat includes providing an illumination sub-system which has an extremeultraviolet soft x-ray source and providing a mask sub-system that has amask receiving member and a reflective mask Ti doped high purity SiO₂glass mask wafer with an unetched glass mask surface coated with areflective multilayer coating having a reflectivity of at least 65% toextreme ultraviolet soft x-rays that is received in the mask receivingmember. The method further includes providing a projection sub-systemincluding a camera with a depth of focus≧1 μm and a numerical apertureNA≦0.1; providing a radiation sensitive print sub-system with aradiation sensitive print media; and aligning the illuminationsub-system, the mask sub-system, the projection sub-system, and theradiation sensitive print sub-system wherein the extreme ultravioletsoft x-ray source illuminates the reflective mask with extremeultraviolet soft x-ray radiation and forms a printing pattern which isprojected by the projection sub-system camera onto said radiationsensitive print media.

The invention further includes a method of making a reflective extremeultraviolet soft x-ray mask which includes the steps of: providing a Tidoped high purity SiO₂ glass preform having a preform surface and freeof inclusions, finishing the preform surface into a planar mask wafersurface with an Ra surface roughness≦0.15 nm, and coating the finishedplanar mask wafer surface with a reflective multilayer coating to form areflective mask surface having a reflectivity of at least 65% to extremeultraviolet soft x-rays.

The invention also includes a reflective extreme ultraviolet soft x-raymask wafer that comprises a Ti doped high purity SiO₂ inclusion-freeglass wafer having an unetched first polished planar face surface and anopposing second polished planar face surface, with the first surfacefree of printable surface defects that have a dimension greater than 80nm and has a Ra roughness≦0.15 nm.

The invention further includes a method of making a reflective extremeultraviolet soft x-ray mask wafer that has the steps of: providing a Tidoped high purity SiO₂ glass preform with a first preform surface and anopposing second preform surface, and is free of inclusions, andfinishing the first preform surface into a planar mask wafer surfacehaving an Ra roughness≦0.15 nm.

Additional features and advantage of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended Figures.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying Figures are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The Figures illustrate various embodimentsof the invention, and together with the description serve to explain theprincipals and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic depiction of an embodiment of the invention;

FIG. 2 is a side cross-section of an embodiment of the invention;

FIG. 3 is a top view of an embodiment of the invention;

FIG. 4 is a diagrammatic depiction of an embodiment of the invention;

FIG. 5A is an AFM photomicrograph of an embodiment of the invention;

FIG. 5B is an AFM photomicrograph of an embodiment of the invention;

FIG. 6 a diagrammatic depiction of an embodiment of the invention;

FIGS. 7A-7F are a manufacturing flow depiction of an embodiment of theinvention; and

FIG. 8 a plot of Thermal Expansion (y-axis) versus Temperature (x-axis)for various wt. % TiO₂ levels in SiO₂ glass.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying Figures. An exemplary embodiment of the projectionlithographic method/system of the present invention is shown in FIG. 1and is designated generally throughout by reference numeral 20.

In accordance with the invention, the present invention for a projectionlithographic method includes providing a mask sub-system illuminated byextreme ultraviolet soft x-ray radiation λ produced by an illuminationsub-system, with the mask sub-system including a patterned reflectivemask for forming a projected mask pattern when illuminated by radiationλ with the patterned reflective mask including a Ti doped high puritySiO₂ glass wafer with a patterned absorbing overlay overlaying areflective multilayer coated Ti doped high purity SiO₂ glass defect freewafer surface that has an Ra roughness≦0.15 nm.

As embodied herein, and depicted in FIG. 1, projection lithographicmethod/system 20 comprises mask sub-system 22 which includes patternedreflective mask 24. As shown in FIGS. 2 and 3, patterned reflective mask24 includes a Ti doped high purity SiO₂ glass wafer 26 with a patternedabsorbing overlay 28 overlaying reflective multilayer coated Ti dopedhigh purity SiO₂ glass defect free wafer surface 30. Reflectivemultilayer coated Ti doped high purity SiO₂ glass defect free wafersurface 30 is comprised of reflective multilayer coating 34 on wafersurface 32, with reflective multilayer coating 34 preferably directlycoating Ti doped high purity SiO₂ glass wafer surface 32. FIG. 4 showsthe optical alignment of projection lithographic method/system 20.

The projection lithographic method for producing integrated circuitswith printed feature dimensions less than 100 nm includes providingillumination sub-system 36 for producing and directing extremeultraviolet soft x-ray radiation λ. Illumination sub-system 36 includesextreme ultraviolet soft x-ray source 38. In a preferred embodiment,illumination sub-system 36 includes a 1.064 μm neodymium YAG laser 40which produces a Xenon gas plasma 42 which outputs extreme ultravioletsoft x-ray radiation λ that is directed by condenser 44. Alternatively,extreme ultraviolet soft x-ray source 38 may comprise a synchrotron,discharge pumped x-ray lasers, an electron-beam driven radiation sourcedevice, or a radiation source based on high harmonic generation based onfemto-second laser pulses.

The projection lithographic method includes providing mask sub-system 22which is illuminated by the extreme ultraviolet soft x-ray radiation λproduced by illumination sub-system 36. Providing mask sub-system 22includes providing a patterned reflective mask 24 which forms aprojected mask pattern when illuminated by the radiation λ. Providingmask 24 includes providing Ti doped high purity SiO₂ glass wafer 26which has a patterned absorbing overlay 28 overlaying reflectivemultilayer coated Ti doped high purity SiO₂ glass defect free wafersurface 30. Ti doped high purity SiO₂ glass wafer surface 30 has an Raroughness≦0.15 nm. Ti doped high purity SiO₂ glass wafer 26 is free ofdefect inclusions and is a noncrystalline amorphous homogeneous glasssolid with wafer surface 30 having a very smooth planar surface that hasan Ra roughness≦0.15 nm and inhibits scattering of radiation λ that isto be reflected by the mask. Such an Ra roughness is achieved byfinishing the surface in accordance with the invention and is measuredusing Atomic Force Microscopy (AFM). FIGS. 5A-5B are AFMphotomicrographs of a Ti doped high purity SiO₂ glass wafer surface ofthe invention which has an Ra roughness≦0.15 as determined by AFMmeasurements.

The method includes providing a projection sub-system 46 and anintegrated circuit wafer 48 with a radiation sensitive wafer surface 50,wherein projection sub-system 46 projects the projected mask patternfrom mask 24 onto radiation sensitive wafer surface 50. Preferablyprojection sub-system 46 is a series of four mirrors as shown in FIGS. 1and 4 which reduce the size of the mask pattern and project the reducedpattern image onto wafer surface 50, with a 4× reduction power.

In a preferred embodiment, extreme ultraviolet soft x-ray radiation λ isin the range from about 5 nm to about 15 nm, and most preferablyillumination sub-system 36 directs radiation centered about 13.4 nm toreflective mask 24 that has a reflectivity of at least 65% at 13.4 nm.

The provided Ti doped high purity SiO₂ glass wafer 26 is defect free inthat the glass body is free of bulk glass defects and is free ofinclusion, including glass voids and gas filled voids in the glass, andis particularly free of any such defects or imperfections that have adimension greater than 80 nm. In a particularly preferred embodiment,glass wafer surface 32 is an unetched glass surface that has beenfinished with polishing to a planar surface Ra roughness≦0.15 nm. Wafer26 Ti doped SiO₂ glass is substantially non-transmissive to extremeultraviolet soft x-ray radiation λ and the reflectivity of reflectivecoated wafer surface 30 and the ultra-low roughness of wafer surface 32is utilized in the invention to inhibit scattering of illuminationradiation and provide an ultra-stable high quality image on wafersurface 50 during the projection lithography process. The provided Tidoped high purity SiO₂ glass is preferably chlorine-free and preferablyhas an impurity level of less than 10 ppb of alkali and alkaline earthmetals. Preferably the Ti doped high purity SiO₂ glass contains from 5to 10 wt. % TiO₂ and has a coefficient of thermal expansion in the rangefrom +30 ppb to −30 ppb at 20° C., and more preferably in the range from+10 ppb to −10 ppb at 20° C. Preferably wafer 26 has a variation incoefficient of thermal expansion throughout the wafer that is less thanor equal to 15 ppb. Wafer 26 preferably has a thermal conductivityK≦1.40 w/(m.° C.) at 25° C., more preferably in the range from 1.25 to1.38, and most preferably about 1.31.

During projection lithography, wafer 26 is heated by the illumination ofradiation λ, and even with such heating of the wafer the dimensions ofthe patterned absorbing overlay is substantially unaffected and changesof the projected image are inhibited and the quality of the projectedimage is maintained. In the inventive method, the Ti doped high puritySiO₂ glass wafer 26 is heated to an operating temperature by radiation λand preferably the glass wafer has an adjusted Ti dopant level adjustedsuch that the glass wafer has a coefficient of thermal expansioncentered about 0 at such operating temperature. With such a thermalconductivity and coefficient of thermal expansion, wafer 26 providesappropriate operation and stability, and provides a highly reliable andeconomic lithographic method/system in which mask sub-system 22 does notrequire cooling. In a preferred embodiment, mask 24 and wafer 26 are notactively cooled and are free of a cooling system such as circulatingcooling fluids, thermal electric coolers, or other means for removingheat build up.

The provided Ti doped high purity SiO₂ glass defect free wafer surface32 has a finished planar surface, with the finished planar surface freeof printable surface imperfections that have a dimension greater than 80nm. Preferably the finished planar surface is free of printable surfaceimperfections that have a dimension greater than ⅕ of the smallestprinted feature dimension on wafer surface 50, such that the finishedplanar surface of the mask does not pollute the image printed on wafersurface 50. The reflective multilayer coated Ti doped high purity SiO ₂glass wafer surface 30 preferably reflects at least 65%, and morepreferably reflects at least 70%, of the radiation λ illuminating thereflective multilayer coating. In a preferred embodiment wafer surface32 is unetched and free of an intermediate barrier layer or releaselayer so that multilayer reflective coating 34 is directly bondedthereto.

The invention further comprises a method of making a projectionlithographic system and the method of projecting lithographic patternsthat includes providing an illumination sub-system 36 with an extremeultraviolet soft x-ray source 38 and providing a mask sub-system 22 witha mask receiving member 52 and a reflective mask Ti doped high puritySiO₂ glass mask wafer 24 with an unetched glass mask wafer surface 32with an Ra roughness≦0.15 nm coated with a reflective multilayer coating34 having a reflectivity of at least 65% to extreme ultraviolet softx-rays received in mask receiving member 52. The method further includesproviding a projection sub-system 46 with a camera 54 having a depth offocus≧1 μm and a numerical aperture NA≦0.1; providing a radiationsensitive print sub-system 56 with a radiation sensitive print media 58;and aligning illumination sub-system 36, mask sub-system 22, projectionsub-system 46, and radiation sensitive print sub-system 56 such thatextreme ultraviolet soft x-ray source 38 illuminate reflective mask 24with extreme ultraviolet soft x-ray radiation and reflective mask 24reflects the radiation and forms a printing pattern of overlay 28 whichis projected and reduced by projection sub-system camera 54 ontoradiation sensitive print media 58.

The method includes providing a reflective mask 24 with a Ti doped highpurity SiO₂ glass mask wafer 26 free of inclusions and wafer surfacedefects that are printable on radiation sensitive print media 58.Providing mask sub-system 22 including a reflective mask 24, includesproviding a Ti doped high purity SiO₂ glass mask wafer preform having apreform surface and free of inclusions, and finishing the preformsurface into a planar mask wafer surface having an Ra roughness≦0.15 nmin accordance with the invention.

The method preferably includes determining an operating temperature ofreflective mask 24 when illuminated by illumination sub-system 36 duringoperation of system 20 and providing a mask sub-system 22 with areflective mask 24 includes providing a Ti doped high purity SiO₂ glassmask wafer having a coefficient of thermal expansion at the determinedoperating temperature centered about 0. Such tuning of the coefficientof thermal expansion is done by controlled variations of Ti dopantcontent in the SiO₂ glass as demonstrated by FIG. 8 which shows thethermal expansion characteristics for high purity SiO₂ glasses with TiO₂wt. % from 0 to 9.0 wt. % TiO₂.

In a further embodiment of the method the Ti doped high purity SiO₂glass mask wafer is heated to a raised temperature range by the extremeultraviolet soft x-ray radiation and the Ti doped high purity SiO₂ glasshas a coefficient of thermal expansion for the raised temperature rangethat is less than 10 ppb and greater than −10 ppb. Preferably the methodincludes providing a mask 24 and wafer 26 that is a thermal insulator(low thermal conductivity) with a thermal conductivity K≦1.40 w/(m.°C.), preferably in the range from 1.25 to 1.38, most preferably about1.31, and allowing wafer 26 to heat up without actively cooling wafer26.

The invention further comprises a method of making a reflective extremeultraviolet soft x-ray projection lithography mask 24 that includesproviding a Ti doped high purity SiO₂ glass preform having a preformsurface and free of inclusions, finishing the preform surface into aplanar mask wafer surface 32 having an Ra roughness≦0.15 nm, and coatingthe finished planar mask wafer surface having a Ra roughness≦0.15 nmwith a reflective multilayer coating 34 to form a reflective masksurface 30 having a reflectivity of at least 65% to extreme ultravioletsoft x-rays. The method further includes forming a patterned absorbingoverlay 28 on reflective mask surface 30. Coating with a reflectivemultilayer coating preferably includes forming alternating smooth thinlayers (≦4 nm thick) of a first element and a second element, such asMo/Si or Mo/Be.

The alternating layers provide a soft x-ray extreme ultravioletreflectivity peak, preferably centered about 13.4 nm. Such analternating multilayer reflective coating functions similarly to aquarter wave stack, with the thickness of the layers optimized forconstructive interference of the photons reflected at each interface andminimal absorption so a large number of interfaces contribute to thehigh reflectance of the coating. Preferably layer to layer variation inthickness is within 0.01 nm. In a preferred embodiment, the coating is81 alternating layers of Mo and Si with Mo thickness about 2.8 nm andthe Si thickness about 4.0 nm. With proper deposition conditions,reflectances of 68% or greater at 13.4 nm are achievable with suchalternating layers. The 81 alternating layers of Mo and Si arepreferably capped with a capping layer to prevent oxidation of the Mo onexposure to normal atmospheres, with a preferred capping layer being a 4nm thick Si layer. The reflective coating 34 of mask 24 is thenpatterned using wafer processing steps to form a pattern in an absorbinglayer deposited on top of the reflective coating.

The absorbing layer is comprised of an absorbing element such as Al, Tior other soft x-ray extreme ultraviolet absorbing elements and ispatterned with wafer processing steps such as optical exposure or e-beamdirect write steps to form patterned absorbing layer 28. In a preferredembodiment, coating with a reflective multilayer coating and forming apatterned absorbing overlay includes, multilayer deposition on the Tidoped high purity SiO₂ glass wafer surface 32 which has an Raroughness≦0.15 nm, then buffer layer deposition on top of multilayerreflective coating, then absorber deposition on top of buffer layer,than pattern generation lithography, then pattern transfer intoabsorber, and then buffer layer etch removal to result in reflectivecoating with patterned absorber layer, with the reflective coatinghaving a reflectivity of at least 65% to extreme ultraviolet softx-rays, preferably centered about 13.4 nm.

In the preferred embodiment, the reflective multilayer coating isdirectly deposited onto the Ti doped high purity SiO₂ glass wafersurface. The finished surface and the Ti doped high purity SiO₂ glassproperties provide for directly depositing and bonding between the glasssurface and the reflective multilayer coating without the complicationsof indirect coating and without the need for additional treatments of asubstrate surface such as smoothing coating processes used withglass-ceramic crystalline containing substrate materials and remove theneed for intermediate layers between the glass surface and thereflective multilayer coating. A superior and stable reflective mask isachieved with the direct bonding contact between the finished Ti dopedhigh purity SiO₂ glass surface and the multilayer reflective coating.

Preferably providing a Ti doped high purity SiO₂ glass preform as shownin FIG. 6, includes providing a high purity Si containing feedstock 114and a high purity Ti containing feedstock 126, delivering high purity Sicontaining feedstock 114 and high purity Ti containing feedstock 126 toa conversion site 100, converting the delivered feedstocks into Ti dopedSiO₂ soot 101, depositing soot 101 in a revolving zircon collection cup142 of refractory zircon furnace 140 onto the upper glass surface of hotTi doped high purity SiO₂ glass body 144, concurrently with the sootdeposition consolidating the Ti doped SiO₂ soot into an inclusion freehomogeneous Ti doped high purity SiO₂ glass body 144 and forming glassbody 144 into a Ti doped high purity SiO₂ glass preform. Preferablyproviding high purity Si containing feedstock 114 and high purity Ticontaining feedstock 126 includes providing a chlorine-free high puritySi containing feedstock and providing a chlorine-free high purity Ticontaining feedstock, converting the chlorine-free feedstocks into achlorine-free Ti doped SiO₂ soot and consolidating the soot into achlorine-free Ti doped SiO₂ glass. Preferably the Si feedstock is asiloxane, preferably a polymethylsiloxane, more preferably apolymethylcyclosiloxane, and most preferably high purityoctamethylcyclotetrasiloxane (Si feedstock comprised of at least 99%octamethylcyclotetrasiloxane). Preferably the Ti feedstock is titaniumalkoxide, and more preferably titanium isopropoxide [Ti(OPri)₄],preferably with Ti feedstock comprised of at least 99% titaniumisopropoxide. A nitrogen inert carrier gas 116 is bubbled throughfeedstocks 114 and 126, and a nitrogen inert carrier gas 118 is added tothe Si feedstock vapor/carrier gas mixture and the Ti feedstockvapor/carrier gas mixture to prevent saturation and facilitate deliveryof the feedstocks to conversion site 100, through distribution systems120 and manifold 122. Preferably the Si feedstock is mixed with the Tifeedstock in manifold 122 to form a homogeneous gaseous Ti doped SiO₂glass precursor mixture which is delivered through conduits 134 toconversion site burners 136 mounted in the upper portion 138 of furnace140 which produce conversion site burner flames 137, so that thefeedstock mixture is converted into Ti doped SiO₂ soot 101 and thenhomogeneous Ti doped SiO₂ glass 144. The weight percent dopant contentof TiO₂ in the SiO₂ glass can be adjusted by changing the amount of Tifeedstock delivered to conversion site 100 that is incorporated intosoot 101 and glass 144. In a preferred method the Ti dopant weightpercent level of glass 144 and preform 60 is adjusted so that wafer 26has a coefficient of thermal expansion centered about 0 at an operatingtemperature of the mask incorporating wafer 26. In accordance with FIG.8, adjustments to the wt. % TiO₂ in the high purity SiO₂ glass adjustthe thermal expansion characteristics of the resultant glass wafer.Preferably the Ti dopant weight percent level of the glass is adjustedwithin the range from about 6 wt. % TiO₂ to about 9 wt. % TiO₂, mostpreferably in the range from 7 to 8 wt. %. Conversion site burner flames137 are formed with a fuel/oxygen mixture (Natural Gas and/or H₂/withO₂) which combusts, oxidizes and converts the feedstocks attemperatures≧about 1600° C. into soot 101 and consolidates the soot intoglass 144. The temperatures of conduits 134 and the feedstocks containedtherein are preferably controlled and monitored to inhibit reactionsprior to flames 137 which may disrupt the flow of the feedstocks andsoot 101 and complicate the manufacturing process of glass 144.Preferably furnace 140 and particularly zircon cup 142 and upper portion138 are made form high purity zircon refractory bricks which are free ofalkali and alkaline earth metals, and other impurities which end tomigrate from the furnace and contaminate glass 144. Such high puritybricks can be obtained using high purity ingredients and calcining thebricks to leach out impurities.

Preferably cup 142 has a circular diameter shape of at least 0.5 meters,more preferably at least 1 meter, so that glass body 144 is acylindrical boule having a respective diameter of at least 0.5 meters,preferably 1 meter and a height of at least 8 cm, preferably at least 10cm, with a preferred height in the range from 12 to 16 cm. Forming glassbody 144 into a Ti doped high purity SiO₂ glass preform 60, preferablyincludes inspecting glass body 144 for any glass defects, such asinclusions, selecting defect free sections of glass body 144 such asdefect free section 62 of glass body 144 shown in FIG. 7A, and removingthe defect free section 62 from 144 as shown in FIG. 7B. Preforms 60 arethen formed from removed defect free section 62 as shown in FIG. 7C.Preform 60 is preferably obtained by core drilling section 62 from body144 and cutting removed section 62 into planar preforms 60 having a topplanar surface 64 and a bottom planar surface 66 of appropriate sizethat allows finishing of planar preforms 60 into a mask wafer with aplanar mask wafer surface having a roughness≦0.15 nm. As shown in FIG.7E, planar preform 60 is finished by polishing a surface of preform 60.Preferably both the top surface 64 and the bottom surface 66 of preform60 are polished with polishing wheels 68 to result in wafer 26.Preferably polishing preform 60 into wafer 26 includes at least twopolishing steps. Preferably preform 60 is polished into a first polishedpreform surface having a roughness ranging from about 0.6 to 1.0 nm andthen the polished preform surface is further polished into the planarmask wafer surface having a roughness≦0.15 nm. Preferably polishingincludes polishing with colloidal particles which are finely distributedparticles in a liquid medium (solution) that do not settle out ofsolution. All of the particles are less than 0.5 microns in size and theentire size distribution of the particles is less than 0.5 microns andare preferably spherically shaped and have a diameter less than 100 nm,preferably in the range from 20 to 50 nm. Preferably the colloidalparticles are comprised of silica, titania, alumina, or ceria. Mostpreferably the invention includes polishing with colloidal silica,preferably colloidal silica doped with titanium.

As shown in FIG. 7E, the polishing steps of the invention comprisepolishing both the top surface 64 and the opposing bottom surface 66concurrently. Preferably polishing wheels 68 include polishing wheelpads that are formed from synthetic polymers and the rotating motion ofthe synthetic polymer wheel surface and the polishing agent remove partsof the preform surfaces 64 and 66 through a combination of chemical andphysical mechanical action to provide the finished smooth surface onwhich the reflective multilayer coating is deposited. Preferably thefinishing includes polishing the preform surface with a cerium oxideabrasive and hard polyurethane pads, then polishing with a cerium oxideabrasive and soft napped polyurethane pads, and then polishing withcolloidal silica and soft polyurethane pads. In a preferred alternativeembodiment the colloidal silica is doped with titanium. Preferablyfinishing includes polishing with an aqueous solution of at least onemetal oxide and then polishing with an alkali aqueous solution ofcolloidal silica. After polishing with such polishing agents, thepreform surfaces are cleaned to remove the polishing agents and providecleaned wafer 26. Preferably the alkali aqueous solution of colloidalsilica is buffered to a pH ranging from 8 to 12, preferably 10 to 12,and most preferably 10 to 11. The colloidal silica in the alkali aqueoussolution through physical action removes any surface corrosion by thealkali solution and removes an continually-formed hydrated surface layeron the Ti doped SiO₂ glass.

The invention further includes reflective extreme ultraviolet soft x-raymask wafer 26. The wafer comprises a Ti doped high purity SiO₂inclusion-free glass wafer that has an unetched first polished planarface surface 32 and an opposing second polished planar face surface 31with the first surface free of printable surface defects that have adimension greater than 80 nm and having a roughness Ra≦0.15 nm. FIGS. 5Aand 5B show such polished Ti doped high purity SiO₂ inclusion-free glasswafer surfaces. Preferably the second opposing surface 31 is also freeof printable surface defects that have a dimension greater than 80 nmand has a roughness Ra≦0.15 nm. Preferably the wafer has a thicknessdimension between the first surface and the second surface of at leastabout 1 mm, more preferably at least 5 mm, and most preferably in theranges of 6 to 12 mm, and more preferably 6 to 8 mm. Preferably the Tidoped high purity SiO₂ inclusion free glass mask wafer is free ofchlorine and has an impurity level of less than 10 ppb of the alkali andthe alkaline earth metals.

The invention also includes a method of making a reflective extremeultraviolet soft x-ray mask wafer that comprises providing a Ti dopedhigh purity SiO₂ glass preform 60 having a first preform surface 64 andan opposing second preform surface 66 free of glass inclusions andfinishing first preform surface 64 into a planar mask wafer surfacehaving a roughness Ra≦0.15 nm. Preferably providing a Ti doped highpurity SiO₂ glass preform includes providing a high purity Si containingfeedstock 114 and high purity Ti containing feedstock 126, deliveringfeedstocks 114 and 126 to conversion site 100, converting feedstocks 114and 126 into Ti doped SiO₂ soot 101, consolidating soot 101 into aninclusion free homogeneous Ti doped high purity SiO₂ glass and formingthe glass into a Ti doped high purity SiO₂ glass preform 60. PreferablySi feedstock 114 is chlorine free and Ti feedstock 126 is chlorine freeso that soot 101 and glass 144 is chlorine free. Preferably finishingfirst preform surface 64 into planar mask wafer surface 32 includespolishing surface 64 into a first polished preform surface having asurface roughness Ra ranging from about 0.6 to 1.0 nm and then furtherpolishing the polished preform surface into a planar mask wafer surfacehaving a roughness Ra≦0.15 nm. Preferably the polishing includespolishing with colloidal silica. In a preferred embodiment the colloidalsilica is doped with titania in a concentration of 4 to 10 wt. %. In amost preferred embodiment the method includes polishing opposing secondpreform surface 66 concurrently with polishing first surface 64. Thefinishing includes polishing the preform surface with an aqueoussolution of at least one metal oxide and polishing the surfaces with analkali solution of colloidal silica. Preferably the preform 60 surfacesare first polished with a cerium oxide abrasive and a hard polyurethanesynthetic polymer blown pad, then with a cerium oxide abrasive and asoft napped polyurethane synthetic polymer pad and then polished withcolloidal silica and a soft polyurethane synthetic polymer pad. In apreferred embodiment the colloidal silica is doped with titanium.Preferably the preform has a thickness>8 mm and the preform is finishedto provide a wafer 26 having a thickness>6mm. The finishing step inaddition to polishing the preform surfaces with polishing agents,includes cleaning the preform surfaces to remove the polishing agents toprovide a clean smooth surface for contact with the reflectivemultilayer coating. Preferably the step of providing a preform includesadjusting the Ti dopant weight percent level of the Ti doped high puritySiO₂ glass preform so that the mask wafer has a coefficient of thermalexpansion centered about 0 at an operating temperature of the reflectiveextreme ultraviolet soft x-ray mask.

The inventive method of making a reflective extreme ultraviolet softx-ray mask wafer provides an economic means for efficientlymanufacturing large quantities of mask wafers which can enable theutilization of extreme ultraviolet soft x-ray projection lithography forthe mass production of integrated circuits with printed featuredimensions less than 100 nm. Additionally, the inventive method ofmaking a reflective mask Ti doped high purity SiO₂ glass wafer 26includes the beneficial steps of inspecting and qualifying the finishedtop wafer surface 32 to ensure that the surface has proper roughness andis defect free, and additionally inspecting and qualifying the finishedbottom wafer surface 31 to determine the roughness and defect freequalifications of the opposing second glass wafer surface. Preferably,AFM is used in such inspection and qualification. This improves theselection and yield of mask wafers for coating and further utilizing aspart of the mask system for extreme ultraviolet soft x-ray projectionlithography.

During use of the patterned reflective extreme ultraviolet soft x-raymask, the Ti doped SiO₂ glass wafer has a lithography operatingtemperature. The operating temperature of the SiO₂ glass wafer include amaximum operating temperature. During the making of the patternedreflective extreme ultraviolet soft x-ray mask, the Ti doped SiO₂ glassis exposed to manufacturing treatment temperatures. The manufacturingtreatment temperatures include elevated temperatures during cutting,machining, finishing, and coating. The manufacturing treatmenttemperatures include a maximum manufacturing temperature. Preferably theTi doped SiO₂ glass wafer is crystallization resistant and has acrystallization temperature T_(crystal) at which crystallization in theglass is induced, with T_(crystal) being substantially greater than themaximum operating temperature and the maximum manufacturing temperature.Preferably T_(crystal) is at least 400° C. higher than the greater ofthe maximum operating temperature and the maximum manufacturingtemperature, more preferably at least 700° C. higher, and mostpreferably at least 800° C. higher. Preferably T_(crystal) is ≧1300° C.,with the maximum operating temperature and maximum manufacturingtemperature not exceeding 500° C. With the Ti doped SiO₂ glass, thecrystallization of the glass is inhibited at the elevated temperaturesexperienced during manufacturing and use of the mask. The glass wafer isthus beneficial in that it has a high temperature crystallizationbehavior.

Additionally, in view of the manufacturing treatment temperatures andthe lithography operating temperatures, the Ti doped SiO₂ glassmaintains its physical dimensions when exposed to thermal cycling. Inthe use and manufacture of the Ti doped SiO₂ glass wafer reflective maskrepeated thermal cycling between lower temperatures and highertemperatures do not substantially charge the physical dimensions of theglass wafer. Preferably the Ti doped SiO₂ glass is resistant to thermalcycling hysteresis, and most preferably is free of thermal cyclinghysteresis when repeatedly cycled (>100 cycles) from the lowestlithography operating temperature to the highest lithography operatingtemperature. Most preferably physical dimensions of the glass wafer donot measurably change when repeatedly cycled from a low temperature to ahigh temperature that are below 300° C. with the low temperatureproximate to 0° C. and the high temperature proximate to 300° C.

In a preferred embodiment, the Ti doped SiO₂ glass wafer has abirefringence resulting from permanent strain in the glass of less than10 nm/cm, and more preferably is less than 2 nm/cm. Preferablybirefringence less than 2 nm/cm is achieved by providing a constanthomogeneous distribution of Ti dopant throughout the SiO₂ glass with thecoefficient of thermal expansion in the range from +10 ppb to −10 ppb at20° C., with the maximum fluctuation in the coefficient of thermalexpansion being less than 10 ppb, most preferably less than 5 ppb.Additionally, low birefringence may be obtained by annealing the Tidoped SiO₂ glass. Preferably the Ti doped SiO₂ glass is annealed at atemperature of at least 900° C., more preferably at least 1000° C., andmost preferably at least 1030° C. after the glass has experiencedstress, such as from machining.

Ensuring such low birefringence levels is preferably achieved bymonitoring the coefficient of thermal expansion of the Ti doped SiO₂glass by transmitting ultrasonic waves through the glass and measuringthe transit time of the ultrasonic waves through the glass to determinethe ultrasonic velocity and expansivity characteristics of the glassexposed to the ultrasonic waves. Preferably such measurement andmonitoring of the Ti doped SiO₂ glass is utilized throughout the maskmanufacturing process. Such ultrasonic measurements are preferablyutilized for quality control, inspection, and selection. Suchmeasurement are preferably utilized to insure the manufacturing of glassbody 144 is providing a constant homogeneous distribution of Ti dopant.Additionally, such measurements are used in inspecting glass body 144and selecting sections 62 to be cut from glass body 144. Further suchmeasurements may be utilized to insure that undue stresses are notformed in the glass during later manufacturing stages such as finishingand also used as a determining factor if further annealing of the glassis required.

EXAMPLE

FIGS. 5A-5B are AFM photomicrographs of a Ti doped high purity SiO₂glass wafer surface. FIGS. 5A-5B were taken at two separate locations onthe same finished wafer sample top surface. The mask wafer sample wasobtained by finishing a Ti doped high purity SiO₂ glass preform that wasfree of inclusions. The preform had a square shape with approximately7.6 cm sides and a thickness of approximately 0.64 cm. The squarepreform was obtained by cutting the preform from an inclusion free areaof a Ti doped high purity SiO₂ glass boule that had a cylindrical shapeof approximately 152 cm diameter and a 14 cm thickness (height). Theboule was produced from octamethylcyclotetrasiloxane and titaniumisopropoxide feedstocks in accordance with FIG. 6 of the invention withthe Ti doped high purity SiO₂ glass having a TiO₂ wt. % of about 7.5 wt.%. The square preform were finished into a planar mask wafer using adouble sided lapping/polishing machine. The square preform was firstlapped using 7 micron alumina abrasives on cast iron plate to removeapproximately {fraction (20/1000)} inches (0.0508 cm) of the preformthickness. Then it was polished on blown synthetic polyurethane hard pad(Rodel Inc., 3804 E. Watkins Street, Phoenix, Ariz.; Rodel MHC-14B brandblown polyurethane pad) for one hour using cerium oxide (Rodia Inc., 3Enterprise Drive, Shelton, Conn.; Rodia (Rhone-Poulence) Opaline brandcerium oxide) at 1.5 psi (0.1055 kg/cm²) and 50 RPMs. The preform wasthen polished on napped synthetic polyurethane soft pad (Rodel Inc.,Rodel 204 Pad brand napped polyurethane pad) for twenty minutes usingcerium oxide (Universal Photonics Inc., 495 W. John Street, Hicksville,N.Y.; Universal Photonics Hastelite 919 brand cerium oxide) at 1.5 psi(0.1055 kg/cm²) and 50 RPMs. The preform was then polished on nappedsynthetic polyurethane soft pad (Rodel 204 Pad brand napped polyurethanepad) for five to ten minutes using colloidal silica (Cabot Corp., 75State Street, Boston, Mass.; Cabot A2095 brand colloidal silica) at 1.5psi (0.1055 kg/cm²) and 50 RPMs. The resulting planar mask wafer wasthen analyzed, measured, and qualified using AFM with the resultingsurface of FIGS. 5A-5B. The finished Ti doped SiO₂ glass wafer surfacehas an Ra roughness≦0.15 nm. In a preferred embodiment the Ti doped SiO₂glass wafer surface of the invention is defect free with an Raroughness≦0.10 nm, more preferably Ra roughness≦0.09 nm, and mostpreferably Ra roughness≦0.086 nm. Additionally, the Ti doped SiO₂ wafersurface preferably has an RMS roughness≦0.15 nm, and average height≦0.5nm with a maximum range≦0.9 nm.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

We claim:
 1. A reflective extreme ultraviolet soft x-ray lithographicmask wafer comprising a Ti doped high purity SiO₂ inclusion-freecrystal-free glass wafer, said Ti doped SiO₂ glass wafer contains from 5to 10 wt. % TiO₂ and a metal impurity level of less than 10 ppm of metalimpurities and has a coefficient of thermal expansion in the range from+10 ppb to −10 ppb at 20° C. with a variation in coefficient of thermalexpansion≦15 ppb, said mask glass wafer having a patterned absorbingoverlay overlaying a reflective multilayer coated Ti doped SiO₂ glassdefect-free finished planar wafer surface that has an Ra roughness≦0.15nm, said reflective multilayer coated Ti doped SiO₂ glass defect-freewafer surface free of printable surface defects that have a dimensiongreater than 80 nm.
 2. A reflective extreme ultraviolet soft x-raylithographic mask wafer as claimed in claim 1 wherein said Ti doped SiO₂glass wafer surface is an unetched glass surface.
 3. A reflectiveextreme ultraviolet soft x-ray lithographic mask wafer as claimed inclaim 1 wherein said reflective multilayer is directly coated on said Tidoped SiO₂ glass wafer surface.
 4. A method of making a reflectiveextreme ultraviolet soft x-ray lithographic mask wafer, said methodcomprising: providing a Ti doped SiO₂ glass preform which contains from5 to 10 wt. % TiO₂ and a metal impurity level of less than 10 ppm ofmetal impurities and has a coefficient of thermal expansion in the rangefrom +10 ppb to −10 ppb at 20° C. with a variation in coefficient ofthermal expansion≦15 ppb and free of inclusions and has a first preformsurface, finishing said first preform surface into a planar mask wafersurface by polishing said first preform surface into a first polishedpreform surface having a surface Ra roughness ranging from about 0.6 to1.0 nm, and further polishing said polished preform surface withcolloidal silica into a defect-free planar mask wafer surface having anRa roughness≦0.15 nm.
 5. A method as claimed in claim 4, said methodincluding coating said finished planar mask wafer surface having an Raroughness≦0.15 nm with a reflective multilayer coating to form areflective mask surface having a reflectivity of at least 65% to extremeultraviolet soft x-rays.
 6. A method as claimed in claim 5, furthercomprising forming a patterned absorbing overlay on said reflective masksurface.
 7. A method as claimed in claim 5 wherein said finished planarsurface is free of printable surface imperfections that have a dimensiongreater than 80 nm.